Pyrophosphate-containing Calcium Phosphates Negatively Impact Heterotopic Bone Quality

In bone, critical size defects pose substantial challenge in maxillofacial and orthopaedic reconstructions as they are incapable of spontaneous regeneration. Autologous bone grafts remain the "gold standard" in bone repair but their limitations and associated morbidities drive the development of bone graft substitute materials. Assessing the osteoinductive properties of these substitutes can help predict their success in bone regeneration applications. Calcium phosphates (CaP) are widely used bone graft substitutes due to their biocompatibility, osteoconductive properties, and potential for osteoinductivity. CaP materials containing monetite, beta-tricalcium phosphate (lower case Greek beta-TCP), and a small amount of calcium pyrophosphate (Ca-PP) possess both osteoconductive and osteoinductive properties. However, the role of Ca-PP in osteoinduction and material degradation remains unexplored. This study investigates heterotopic bone formation in response to five CaP material compositions, maintaining a constant monetite and lower case Greek beta-TCP ratio, with varying amounts of Ca-PP (0 - 12.5%). Twelve adult female sheep (Ovis aries) were subcutaneously implanted with constructs made of six CaP tiles interconnected by a Ti6Al4V frame and a control implant. Histological analysis, backscattered electron scanning electron microscopy, and Raman spectroscopy of samples retrieved at 12- and 52 weeks reveal that Ca-PP does not hinder heterotopic bone formation and minimally impacts CaP degradation. While monetite and lower case Greek beta-TCP transform into apatite, the Ca-PP phase remains unchanged. The addition of Ca-PP to the CaP material influences heterotopic bone quality and inflammatory response during tissue regeneration.